To meet ever increasing performance demands such as tear and puncture resistance, gas impermeability, sealability and clarity, modern packaging films can be quite complex in terms of both their structure (e.g. multilayer) and composition (e.g. type of polymer(s)).
Due to its excellent physical and mechanical properties, processability and clarity, polyethylene is used extensively in the manufacture of packaging films. However, polyethylene is to date ultimately derived from crude oil, and there is now a concerted effort in the packaging industry to avoid or at least reduce the use of such oil based polymers in favour of sustainable, bio-derived alternatives.
Much of the research to date in developing such sustainable, bio-derived alternatives has focussed on utilising naturally occurring bio-polymers such as starch. Starch is an attractive alternative in that it is derived from renewable resources (i.e. plant products), readily available and relatively inexpensive.
A number of approaches have been adopted for incorporating starch into polyethylene based films. For example, monolayer and multilayer films have been prepared using a melt blend of polyethylene and starch. Although such film products comprise a reduced polyethylene content, their manufacture can be problematic. For example, the relatively hydrophilic nature of starch is inherently incompatible with the relatively hydrophobic nature of polyethylene. Accordingly, starch has a tendency to migrate to the surface of a starch/polyethylene meltstream during melt processing and collect at certain parts of the processing equipment such as the die lips. Starch or oxidised residue thereof deposited on the die lips can then intermittently pick off into passing film causing holes and defects in the resulting film product.
Furthermore, the incompatibility of starch within a polyethylene matrix typically results in the formation of a multi-phase morphology having a high interfacial tension that often negatively impacts on the physical and mechanical properties of a resulting polymer film. For example, the presence of starch within a polyethylene film can promote significant reductions in the films gloss, elongation properties, toughness, tear strength, puncture resistance and clarity.
An opportunity therefore remains to address or ameliorate one or more disadvantageous or shortcomings associated with conventional film products comprising polyethylene and starch, or to at least provide a useful alternative.